Impacts of ocean acidification on the Reef

In the long-term, ocean acidification is likely to be the most significant impact of a changing climate on the Great Barrier Reef ecosystem.

The oceans absorb carbon dioxide (CO2) from the atmosphere and are estimated to have absorbed about half the excess CO2 released by human activities in the past 200 years.

About half of this anthropogenic CO2 (carbon dioxide in the atmosphere resulting from human activities, such as the burning of fossil fuels, rather than natural processes) is in the upper 10 per cent of oceans (less than 1000 metres depth) due to slow ocean mixing processes.

This absorbed CO2 is resulting in chemical changes in the ocean, and is estimated to have caused a decrease in oceanic pH of 0.1. This is referred to as ocean acidification as the oceans are becoming more acidic (though technically they are still alkaline).

As carbon dioxide (CO2) is absorbed from the atmosphere it bonds with sea water forming carbonic acid. This acid then releases a bicarbonate ion and a hydrogen ion. The hydrogen ion bonds with free carbonate ions in the water forming another bicarbonate ion.

That carbonate would otherwise be available to marine animals for making calcium carbonate shells and skeletons. So the more dissolved carbon dioxide in the ocean, the less free carbonate ions available for making calcium carbonate.

From a current pH of 8.2 (alkaline), it is predicted that the ocean’s pH could fall to about 7.8 (still slightly alkaline) by 2100.

Although the chemistry of ocean acidification is simple and well understood, its effect on marine life is much less well-known as the process has only been recognised for less than a decade. Even relatively small increases in ocean acidity decrease the capacity of corals to build skeletons, which in turn decreases their capacity to create habitat for the Reef's marine life.